Causes of crabs (pubic lice)

By Dr Ananya Mandal, MD

Crabs or pubic lice, contrary to common belief, are not caused by only poor personal hygiene. They may be transmitted to a healthy and clean person through close contact with an infected person. The lice crawl from the hair of one person to the hair of another person. These lice are unable to fly or jump. 1-7

Causative organism and its life cycle

Pubic lice called Pthiris pubis are different from head louse. Adults are yellowish grey or tan and appear like crabs.

Pubic lice have six legs front legs are very large and look like the pincher claws of a crab. Their second and third pairs of legs have claw like ends that helps them hold on to the coarse hairs tightly.

The adult louse is around 2mm in length. Females are usually larger than males. The lice live for one to three months. The female louse in their lifetime may lay up to 300 eggs.

The female lays the eggs and attaches them with specially secreted glue or cementing material to the base of the hairs. The eggs appear whitish grey and are smaller than a pin head. The eggs hatch after 6 to 10 days.

The eggs develop into nymphs. These are immature lice that hatch from the nits. A nymph looks like an adult pubic louse but it is smaller. These take around two to three weeks to mature into adults. To live, a nymph must feed on human blood. They grow to adulthood within two to three weeks and may start their own reproductive cycle.

The pubic louse feeds on human blood. When not on a human body, pubic lice can live for up to 24 hours. They rarely leave the human body except for moving onto another person. These lice live on thick coarse hairs like the pubic hair.

These lice may also live on other body hair like those around the anus, underarms, beard, moustache, chest, abdomen, and, rarely eyebrows and even eyelashes.

They are never found on hair over the scalp since this hair is thinner, softer and finer in comparison to body hair. Pubic lice are different from head lice (called Pediculus humanus capitis). Pubic lice do not live on other animals.

Mode of transmission of the pubic lice

Pubic lice are most commonly passed on through close contact with an infected person. This may be sexual contact as well. Unprotected vaginal, oral or anal sex may transmit the infection from an affected person.

Condoms and other barrier contraception cannot prevent transmission of the lice since these live on the pubic hairs.

Kissing a person whose beard is infested with the lice or hugging them may also spread the lice from person to person. Both men and women can catch them and transmit these lice.

Although rare, pubic lice may also spread by sharing bed linen, clothing, towels, toilet seats etc. with an infected person. This mode of transmission is rare since the lice seldom leave the human they are primarily living to live on an inanimate object like clothing for even and short period of time.

Spread through a shared toilet seat is also extremely rare since the lice cannot live long away from a warm human body and their feet are unable to walk on smooth surfaces such as toilet seats.

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